Steel sheets for automobile face strong request of gauge down by using HSS in view of improving fuel consumption for the environmental conservation. Since many of automobile parts are fabricated by press forming into complex shapes, there are requested materials having high strength and having both high elongation and stretch-flange formability, both of which are indexes of formability.
Steel sheets in recent years increase in the strength than ever, and those having higher than 980 MPa of strength are wanted. In addition, from the point of further weight reduction, the steel sheets are decreasing their thickness, and the request for thin gauge steel sheets of 2.5 mm or smaller thickness increases.
There are proposed various kinds of that type of steel sheets. For example, JP-A-6-172924, (the term “JP-A” referred to herein signifies the “Unexamined Japanese Patent Application Publication”), proposes a steel sheet having excellent stretch-flange formability, in which a bainitic ferrite structure having high dislocation density is formed. Since, however, the steel sheet contains a bainitic ferrite structure of high dislocation density, it has a drawback of poor elongation. In addition, to form the bainitic ferrite, high cooling rate on a runout table is unavoidably necessary. When manufacturing thin gauge steel sheets, therefore, a problem of prevention of meanders of strip on the runout table arises during manufacturing thin gauge sheets so that the technology is not suitable for manufacturing thin gauge sheets of 2.5 mm or smaller thickness.
JP-A-6-200351 proposes a steel sheet having excellent stretch-flange formability giving 70 kg/cm2 or higher tensile strength by adjusting most part of the microstructure to polygonal ferrite and by precipitation strengthening mainly by TiC and solid-solution strengthening. It is, however, difficult to attain high tensile strength of 980 MPa or more by the widely known precipitate used in the steel sheet.
That is, when a large quantity of Ti is added to increase the tensile strength for the purpose of attaining 980 MPa or more, coarse precipitate likely forms, and the desired strength cannot be attained. In addition, increased adding quantity of Ti increases the necessary slab-heating temperature for dissolving TiC into the form of solid solution, thus it tends to become difficult to manufacture the steel sheet by an ordinary apparatus.
JP-A-2004-143518 proposes a hot-rolled steel sheet which contains ferrite having 1 to 5 μm of average grain size as the main phase and which is precipitation-strengthened by carbonitride of V having 50 nm or smaller average particle size. To obtain fine V precipitate, however, there is generally needed the coiling at a low temperature of 550° C. or below. As a result, increase in the quantity of precipitate becomes difficult, and the strengthening has a limitation. Therefore, with the steel sheet, combination with grain refinement strengthening of ferrite, described above, is required to achieve higher tensile strength.
In the technology described in JP-A-2004-143518, however, the refinement of ferrite grains needs, in the finish rolling step, the rolling of sheet at Ar3 transformation point or higher temperature at a rolling stand before the last stand in the tandem rolling mill row, and then cooling the sheet to a temperature of “Ar3 transformation point—50° C.” or below at an average cooling rate of 50° C./s or more, followed by rolling to 20% or smaller reduction at the final stand. With an ordinary manufacturing line, however, realization of that manufacturing condition is difficult.
Furthermore, since the steel sheet allows the formation of pearlite and the like, elongation and stretch-flange formability may be deteriorated.
As the technology to obtain an ultrahigh tensile steel sheet, JP-A-2002-322539 and JP-A-2003-89848 disclose a technology to manufacture ultrahigh tensile steel sheet having both excellent elongation and stretch-flange formability by dispersing fine carbide consisting of C, Ti, and Mo into the ferrite single phase. Similar to the technology disclosed in JP-A-6-200351, however, when a large quantity of C and Ti is added to obtain 980 MPa or higher tensile strength, normal slab-heating temperatures (about 1150° C. to about 1250° C.) cannot completely dissolve TiC and other substances precipitated in the slab, in some cases. That is, to completely dissolve TiC and other substances for attaining high strength, further high temperature is required, which makes manufacturing the steel difficult in some cases, and, even if the manufacturing is conducted, a heavy load is applied to the manufacturing apparatus.